The range of years included is different for each collection and because of that some records give better information than others. For example the top record below is a record form the New York collection for a marriage from 1914. These records were gathered from the various counties who had started to require these records.
The record below is from the collection for Louisiana from 1870. The original records are in the possession of the Louisiana State Archives.
Most of the records from the United States are from the time frame that will also include the majority of census records, which makes these records vital to verifying family connections. The collections are not complete and will continue to be added to as more records are made available.
Even with being incomplete, these are a wonderful resource for family historians and as with all other FamilySearch collections they are available for no charge at www.familysearch.org.